Record carrier with a servo track having wobble with reduced cross-talk and an apparatus for scanning the record carrier

ABSTRACT

A record carrier ( 1 ) has a servo track ( 4 ) indicating an information track ( 9 ) intended for recording information blocks represented by marks, this servo track ( 4 ) having a periodic variation of a physical parameter. Due to cross-talk, there exists a variation in the wobble signal called wobble beat. The track pitch t and the wobble period p, p being the length of a wobble period, are chosen in such a way that 2πt/p≈n+½ where n is integer. The choice results in a reduced wobble beat. A recording and/or playback device has apparatus for recording/reading the information blocks and for generating a wobble signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.10/131,776, filed Apr. 24, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a record carrier comprising a servo trackindicating an information track intended for recording informationblocks represented by marks having lengths expressed in channel bits,this servo track having a wobble being a periodic variation of aphysical parameter, and the servo track constituting a concentric orspiral pattern of substantially parallel tracks at a track pitch t.

The invention further relates to recording and/or playback devicecomprising means for writing and/or reading information blocksrepresented by marks having lengths expressed in channel bits in aninformation track on the record carrier, which device comprises meansfor scanning the servo track and retrieving the record carrierinformation.

The invention further relates to a method for manufacturing the recordcarrier.

2. Description of the Related Art

A record carrier and device of the type defined in the openingparagraph, for reading and/or writing information, are known fromInternational Patent Application No. WO 00/43996, corresponding to U.S.Pat. Nos. 6,538,982 and 6,765,861 (Attorney Docket No. PHN 17323). Theinformation is encoded into an information signal which includes timecodes and may be subdivided in accordance with these time codes intoinformation blocks, the time codes being used as addresses, such as,with CD-ROM or DVD+RW. The record carrier has a servo track, usuallycalled pre-groove, for causing servo signals to be generated whenscanning the track. A physical parameter, e.g., the radial position, ofthe pre-groove periodically varies constituting a so-called wobble.During the scanning of the track, this wobble leads to a variation ofthe servo signals and a wobble signal can be generated.

A problem of the known system is that the wobble signal is disturbed bycross-talk. This problem become increasingly important in newer opticalrecording systems, such as, DVD and DVR, because the tracks are closertogether in these higher density systems (even when scaling with theoptical parameter is taken into account), and thus, the cross-talkbetween tracks increases.

SUMMARY OF THE INVENTION

It is an object of the invention, for example, to provide a recordcarrier and device in which the effect of cross-talk in the wobblesignal is reduced.

According to the invention, a record carrier, as defined in the openingparagraph, is characterized in that the track pitch t and the length ofa wobble period p are related so that 2πt/p≈n+½, where n is an integer.Further, the recording and/or playback device, as described in theopening paragraph, is characterized in that the device comprises meansfor generating a wobble signal from the record carrier as describedabove.

The invention is based on the following recognition. Many opticalrecording formats contain a wobble for write-clock generation. Thiswobble is usually predominantly monotonic to reduce write-clock jitter.In formats with high data density such as DVD and DVR, the tracks areclose together. This implies that the spot on a central track not onlysees the signal from the wobble on that central track, but also thesignals of the wobbles on the adjacent tracks. In formats with constantlinear density, such as DVD and DVR groove-only, the frequencies of thewobbles on the adjacent tracks as seen by the spot on a central trackare slightly different from the frequency of the wobble on the centraltrack due to the slightly different radius of the different tracks. Thiscombination of cross-talk and frequency difference causes wobble beat,i.e., a slow variation in both the amplitude and the phase of thewobble. Details of the calculation of wobble beat are given in thedescription. Wobble beat can be a problem. On the one hand, the wobblesignal should be sufficiently large for robust detection of the wobble.On the other hand, the wobble signal should be sufficiently small so asnot to distort the high-frequency data. In the ideal situation, wobblebeat would be absent. To reduce the amplitude of the wobble beat, thetrack pitch t and the wobble period p (i.e., the number of channelbits * the length of a channel bit) are chosen in such a way that2πt/p≈n+½ where n is integer.

A further embodiment of the record carrier is characterized in that therelation of wobble period and track pitch is 0.30<2πt/p−n<0.70. It isnot always possible to select the optimal value of ½ defined above. Thisbecause of restrictions resulting from the disc format (e.g., the totalwobble length should comprise an integer number of wobbles, there shouldbe an integer number of wobbles in a recording unit block, etc.). Inparticular, the value n=0 is a suitable choice, because then the wobbleperiod is at its maximum.

A further embodiment of the record carrier is characterized in that thelength of the wobble period p corresponds to a whole number m times thelength of a channel bit. Such fixed relation has the advantage that thewriting process can be easily locked to the detected wobble signal. Anexample of an appropriate relation is, e.g., a track pitch of 320 nm, achannel bit length of 80 nm and a wobble length of 69 times the channelbit length resulting in a value of (2·π·320 nm)/(69·80 nm)=0.364.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from andelucidated further with reference to the embodiments described by way ofexample in the following description and with reference to theaccompanying drawings, in which:

FIGS. 1 a-1 d show a record carrier provided with a servo-pattern;

FIG. 2 shows the wobble beat;

FIG. 3 shows a device for generating a bit clock;

FIG. 4 shows a device for writing information blocks;

FIG. 5 shows a device for reading information blocks; and

FIG. 6 shows calculated beat modulation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the Figures, elements which correspond to elements already describedhave the same reference numerals.

FIG. 1 a shows a disc-shaped record carrier 1 provided with a track 9intended for recording and a central hole 10. The track 9 is arranged inaccordance with a spiral pattern of windings 3. FIG. 1 b is across-section taken on the line b-b of the record carrier 1, in which atransparent substrate 5 is provided with a recording layer 6 and aprotective layer 7. The recording layer 6 may be optically writable, forexample, via phase change, or magneto-optically writable by a device forwriting information, such as the known CD-Rewritable or CD-Recordable.The recording layer may also be provided with information via aproduction process, in which, first, a master disc is made, and thenthis master disc is subsequently replicated through pressing. Theinformation is organized in information blocks and is represented byoptically readable marks in the form of a succession of areas reflectingmuch radiation and little radiation such as, for example, a successionof pits of different lengths in a CD. In one embodiment, the track 9 onthe record carrier of a rewritable type is indicated by a servo-patternwhich is provided during manufacture of the blank record carrier. Theservo-pattern is formed, for example, by a pre-groove 4 which enables awrite head to follow the track 9 during scanning. The pre-groove 4 maybe implemented as a deeper or a raised part, or as a material propertydeviating from its ambience. Alternatively, the servo-pattern mayconsist of an alternation of elevated and deeper windings, referred toas land and groove patterns, with a transition from land to groove orvice versa taking place per winding. FIGS. 1 c and 1 d show two examplesof a periodical modulation (wobble) of the pre-groove. This wobbleproduces an extra signal in a tracking servo-sensor. The wobble is, forexample, frequency-modulated, and position information, such as, anaddress, a time code or winding information, is coded in the modulation.A description of a rewritable CD system which is provided with positioninformation in this way can be found in U.S. Pat. No. 4,901,300 (PHN12.398) and U.S. Pat. No. 5,187,699 (PHQ 88.002). A servo-pattern mayalso consist of, for example, regularly distributed sub-patterns whichperiodically cause tracking signals. This description is based oninformation storage in a spiral track pattern which is filled from theinner winding to the outer winding, for example, as in a CD-ROM.

FIG. 2 shows wobble beat. It is clearly visible that the top trace, forwhich 2πt/p is closest to 0.5, shows the smallest wobble beat.

In the following, the calculation of wobble beat is outlined. Assumingthe following simple model for cross-talk between wobbles of adjacenttracks:I _(PP) =a ₀ cos(2πs ₀ /p)+a ₁ cos(2πs ₁ /p)+a ⁻¹ cos(2πs ⁻¹ /p),where I_(PP) is the push-pull signal, a_(i) (i=−1, 0, 1) are theamplitudes of the wobble signals of the central (i=0) and the adjacent(i=−1, 1) tracks as measured by the spot on the central track, s_(i)(i=−1, 0, 1) are the total lengths of the tracks i from the inner radiusup to the position under consideration, and p is the period of thewobble.

Furthermore, it is assumed that the track is an ideal spiral. Then, theposition along the track is fully characterized by either the lengthalong the track s, the radius r, or the angle φ. If the spiral has atrack pitch t and starts at radius r_(b), the following relation holdfor an ideal spiral:${s = {{\pi\left( {r^{2} - r_{b}^{2}} \right)}/t}},{\varphi = {2{{\pi\left( {r - r_{b}} \right)}/t}}},{r = {{{\frac{\varphi}{2\pi}t} + r_{b}} = {\frac{\varphi + \varphi_{b}}{2\pi}t}}},{r_{b} = {\frac{\varphi_{b}}{2\pi}{t.}}}$

In practice, a spiral on a disc will never be ideal. It is, however,sufficient that the spiral is close to ideal locally, i.e., on thelength scale of a few revolutions. The formulas of the ideal spiral canthen be applied, with the restriction that the results should not dependon the precise values of r_(b) and φ_(b). Results that depend on r_(b)or φ_(b) would require the spiral to be ideal across the entire disc.Using the above formulas, the following results are obtained for thetotal lengths of the central and adjacent tracks:s ₀=π(r ₀ ² −r _(b) ²)/t,s ₁=π((r ₀ +t)² −r _(b) ²)/t=s ₀+2πr ₀ +πt=s ₀+2π(r ₀ +t/2)=s₀+(φ₀+φ_(b)+π)t,s ⁻¹≦π((r ₀ −t)² −r _(b) ²)/t=s ₀−2πr ₀ +πt=s ₀−2π(r ₀ −t/2)=s₀−(φ₀+φ_(b)−π)t.

By combining the above formulas, the following expression for the wobblebeat signal are obtained: $\begin{matrix}{I_{PP} = {{a_{0}{\cos\left( {2\pi\quad{s_{0}/p}} \right)}} + {a_{1}{\cos\left( {2\pi\quad{s_{1}/p}} \right)}} + {a_{- 1}{\cos\left( {2\pi\quad{s_{- 1}/p}} \right)}}}} \\{= {{Re}\left\{ {{a_{0}{\mathbb{e}}^{{\mathbb{i}2\pi}\quad{s_{0}/p}}} + {a_{1}{\mathbb{e}}^{{\mathbb{i}2\pi}\quad{s_{1}/p}}} + {a_{- 1}{\mathbb{e}}^{{\mathbb{i}2\pi}\quad{s_{- 1}/p}}}} \right\}}} \\{= {{Re}\left\{ {a_{0}{{\mathbb{e}}^{{\mathbb{i}2\pi}\quad{s_{0}/p}}\left( {1 + {\frac{a_{1}}{a_{0}}{\mathbb{e}}^{{{\mathbb{i}2\pi}{({s_{1} - s_{0}})}}/p}} + {\frac{a_{- 1}}{a_{0}}{\mathbb{e}}^{{{\mathbb{i}2\pi}{({s_{- 1} - s_{0}})}}/p}}} \right)}} \right\}}} \\{= {{Re}\left\{ {a_{0}{{\mathbb{e}}^{{\mathbb{i}2\pi}\quad{s_{0}/p}}\left( {1 + {\frac{a_{1}}{a_{0}}{\mathbb{e}}^{{{\mathbb{i}2\pi}{({s_{1} - s_{0}})}}/p}} + {\frac{a_{- 1}}{a_{0}}{\mathbb{e}}^{{{\mathbb{i}2\pi}{({s_{- 1} - s_{0}})}}/p}}} \right)}} \right\}}} \\{= {{Re}\left\{ {a_{0}{{\mathbb{e}}^{{\mathbb{i}2\pi}\quad{s_{0}/p}}\left( {1 + {\frac{a_{1}}{a_{0}}{\mathbb{e}}^{{{\mathbb{i}2\pi}{({\varphi_{0} + \varphi_{b} + \pi})}}{t/p}}} + {\frac{a_{- 1}}{a_{0}}{\mathbb{e}}^{{- {{\mathbb{i}2\pi}{({\varphi_{0} + \varphi_{b} - \pi})}}}{t/p}}}} \right)}} \right\}}}\end{matrix}$

Note that a separation is made between the rapidly varying wobble signal(outside round brackets) and the slowly varying beat (inside roundbrackets).

The additional assumption is now made that the cross-talks from the leftand right tracks are equal, i.e., a⁻¹=a₁. The beat signal can then bewritten as: $\begin{matrix}{\begin{matrix}{1 + {\frac{a_{1}}{a_{0}}{\mathbb{e}}^{{{\mathbb{i}2\pi}{({\varphi_{0} + \varphi_{b} + \pi})}}{t/p}}} +} \\{\frac{a_{- 1}}{a_{0}}{\mathbb{e}}^{{- {{\mathbb{i}2\pi}{({\varphi_{0} + \varphi_{b} - \pi})}}}{t/p}}}\end{matrix} = {1 + {\frac{a_{1}}{a_{0}}{{\mathbb{e}}^{{\mathbb{i}2\pi\pi}\quad{t/p}}\left( {{\mathbb{e}}^{{{\mathbb{i}2\pi}{({\varphi_{0} + \varphi_{b}})}}{t/p}} + {\mathbb{e}}^{{- {{\mathbb{i}2\pi}{({\varphi_{0} + \varphi_{b}})}}}{t/p}}} \right)}}}} \\{= {1 + {2\frac{a_{1}}{a_{0}}{\cos\left\lbrack {\left( {\varphi_{0} + \varphi_{b}} \right)\frac{2\pi\quad t}{p}} \right\rbrack}{\mathbb{e}}^{{\mathbb{i}2\pi\pi}\quad{t/p}}}}} \\{= {1 + {2\frac{a_{1}}{a_{0}}{\cos\left\lbrack {\left( {\varphi_{0} + \varphi_{b}} \right)\frac{2\pi\quad t}{p}} \right\rbrack}{\cos\left( {\pi\frac{2\pi\quad t}{p}} \right)}} +}} \\{i\quad 2\frac{a_{1}}{a_{0}}{\cos\left\lbrack {\left( {\varphi_{0} + \varphi_{b}} \right)\frac{2\pi\quad t}{p}} \right\rbrack}{\sin\left( {\pi\frac{2\pi\quad t}{p}} \right)}} \\{= {\sqrt{\begin{matrix}{1 + {4\frac{a_{1}}{a_{0}}{\cos\left\lbrack {\left( {\varphi_{0} + \varphi_{b}} \right)\frac{2\pi\quad t}{p}} \right\rbrack}{\cos\left( {\pi\frac{2\pi\quad t}{p}} \right)}} +} \\{4\left( \frac{a_{1}}{a_{0}} \right)^{2}{\cos^{2}\left\lbrack {\left( {\varphi_{0} + \varphi_{b}} \right)\frac{2\pi\quad t}{p}} \right\rbrack}}\end{matrix}}*}} \\{\exp\left\lbrack {{\mathbb{i}}\quad{\arctan\left( \frac{2\frac{a_{1}}{a_{0}}{\cos\left\lbrack {\left( {\varphi_{0} + \varphi_{b}} \right)\frac{2\pi\quad t}{p}} \right\rbrack}{\sin\left( {\pi\frac{2\pi\quad t}{p}} \right)}}{1 + {2\frac{a_{1}}{a_{0}}{\cos\left\lbrack {\left( {\varphi_{0} + \varphi_{b}} \right)\frac{2\pi\quad t}{p}} \right\rbrack}{\cos\left( {\pi\frac{2\pi\quad t}{p}} \right)}}} \right)}} \right\rbrack}\end{matrix}$The beat gives rise to both amplitude (the square-root factor) and phasemodulation (the exponential factor). One also easily sees from theseexpressions that the period of one beat equals p/2πt periods. In DVD+RW,for example, p=4265.6 nm and t=740 nm, so it takes 0.917 revolutions tocomplete one beat period.

The maximum and minimum values for the amplitude of the beat are reachedfor:${\sin\left\lbrack {\left( {\varphi_{0} + \varphi_{b}} \right)\frac{2\pi\quad t}{p}} \right\rbrack} = {\left. 0\Leftrightarrow{\cos\left\lbrack {\left( {\varphi_{0} + \varphi_{b}} \right)\frac{2\pi\quad t}{p}} \right\rbrack} \right. = {\pm 1}}$and also for${\cos\left\lbrack {\left( {\varphi_{0} + \varphi_{b}} \right)\frac{2\pi\quad t}{p}} \right\rbrack} = {- \frac{\cos\left( {\pi\frac{2\pi\quad t}{p}} \right)}{2{a_{0}/a_{1}}}}$provided that${{\cos\left( {\pi\frac{2\pi\quad t}{p}} \right)}} < {2{\frac{a_{1}}{a_{0}}.}}$It then follows that the maximum of the beat amplitude is given by:$\sqrt{1 + {4\frac{a_{1}}{a_{0}}{{\cos\left( {\pi\frac{2\pi\quad t}{p}} \right)}}} + {4\left( \frac{a_{1}}{a_{0}} \right)^{2}}},$and the minimum by either of the following two expressions:$\sqrt{1 + {4\frac{a_{1}}{a_{0}}{{\cos\left( {\pi\frac{2\pi\quad t}{p}} \right)}}} + {4\left( \frac{a_{1}}{a_{0}} \right)^{2}}},{{{if}\quad{{\cos\left( {\pi\frac{2\pi\quad t}{p}} \right)}}} > {2\frac{a_{1}}{a_{0}}}},\sqrt{1 - {\cos^{2}\left( {\pi\frac{2\pi\quad t}{p}} \right)}},{{{if}\quad{{\cos\left( {\pi\frac{2\pi\quad t}{p}} \right)}}} < {2{\frac{a_{1}}{a_{0}}.}}}$

From the above expressions, it is clear that the wobble beat is smallestwhen |cos(π2πt/p)| is minimum, i.e., when 2πt/p=n+½, where n is aninteger.

FIG. 3 shows a device for generating a bit clock in accordance with theinvention on the basis of the servo pattern of the record carrier 1,which is rotated by means of a motor 31. The motor 31 may rotate at afixed speed, or the rotational speed may be controlled on the basis ofthe servo pattern. The track is scanned in the conventional manner by aread head 32 via a beam of electromagnetic radiation. During reading,the servo pattern is scanned and a servo-signal is generated forcontrolling the position of the read head by servo-signal generator (notshown). In the device, disc and winding information may be regained fromthe servo-signal by the demodulator 33, for example, by demodulating amodulation of the wobble signal. The modulation may comprise addresses,or some indication of the winding. The winding information comprises,for example, a winding number N_(t) of the relevant winding and,possibly, also a number N_(s) of the relevant segment, these numbersbeing passed on to a computing unit 34. Secondly, synchronizingelements, for example, the pulses in the wobble signal, are detectedfrom the servo-signal by a pulse detector 36. These detected pulses arecoupled to a phase-locked loop (PLL) circuit 37 in which they arecompared with a fraction of the bit clock 38 at the output of the PLL37. This fraction is constituted by a divider 35 which divides the bitclock 38 by an adjustable dividend X. The dividend X may be a fixedvalue, or may be computed by the computing unit 34 from the positiondata (winding number N_(t) and possible segment number N_(s)) and thetrack pitch which is known, for example, from a standard or which ispresent in the disc information on the record carrier. The dividend Xcan be computed once per winding and adjusted in the divider 35. Inanother embodiment, the dividend X may be predetermined or calculatedless frequently or more frequently, for example, per segment or zone.The more frequently the dividend is adjusted, the more accurate the bitclock has the frequency associated with the radial position and, hence,the more accurately the bit length is constant. During continuousscanning of a zoned disc, it is desirable to render the steps in thechange of the bit clock as small as possible. The dividend X may also becomputed in advance for a desired position on the basis of a desiredaddress, for example, when performing a jump instruction. In that case,the computing unit of a system control unit will acquire the informationabout the winding number and possible segment number. It is then anadvantage that, when the jump is being performed, the bit clock canalready be set to the new value which it should have upon arrival at thedesired radial position.

FIG. 4 shows a device for writing information blocks on a disc-shapedrecord carrier of a type which is rewritable in, for example, amagneto-optical or optical manner (via phase change) by means of a beamof electromagnetic radiation. The record carrier is identical to therecord carrier shown in FIGS. 1 a-1 d. During the writing operation,marks representing the information are formed on the record carrier. Thedevice is provided with drive means 45 for rotating the record carrier1, and a write head 42 for scanning the track. The device is furtherprovided with a system control unit 46 for receiving commands from acontrolling computer system or from a user, and for controlling thedevice. To this end, the system control unit comprises, for example, amicroprocessor, a program memory and control gates for performing theprocedures described below, and for controlling said elements. Thesystem control unit 46 may also be implemented as a state machine inlogic circuits. The write head 42 is positioned in the radial directionon the track by positioning means 44, with position information from thetrack being detected. In accordance with known tracking and focusingmethods, the write head scans the track, with, for example, a modulationbeing present in the tracking signal due to a wobble in a servo-pattern.The tracking signal is demodulated and the position information codedtherein is regained in the positioning means 44 and passed on to thesystem control unit 46. The radial position of the write head can beverified by means of the regained position information. The informationpresented to the input of the writing means 41 is distributed, ifnecessary, into information blocks and converted into a write signal forthe write head 42. The writing means 41 comprises, for example, an errorcoder and a channel coder. According to the invention, the writingdevice is provided with clock means 30 for generating the bit clock asdescribed above with reference to FIG. 3, this bit clock being coupledto the writing means 41. The system control unit 46 controls thepositioning means 44, the writing means 41 and the drive means 45, andis equipped for computing the winding numbers and angular positionwithin the winding on the basis of the address of an information block.The system control unit 46 performs this computation in full bit lengths(and possibly simple rational fractions) without rounding errors beingproduced.

FIG. 5 shows a reading device, according to the invention, for readinginformation blocks. The reading device is provided with drive means 45for rotating the disc-shaped record carrier 1, and a read head 52 forscanning the track on the record carrier. The read head 52 is positionedin the radial direction on the track by positioning means 44 on thebasis of signals derived from marks of the record carrier. In accordancewith the conventional Differential Phase Detection or Differential TimeDetection system (DPD or DTD), for example, the reflected radiation canbe received on a detector (not shown), this detector being subdividedinto 4 sub-detectors. By determining phase or time differences betweenthe signals of the sub-detectors, the position of the scanning spot withrespect to the series of marks recorded in the track can be determined.During reading, the signal of the read head 52 is converted into theinformation in the reading means 43, for example, comprising a channeldecoder and an error corrector. According to the invention, the deviceis provided with clock means 30 for generating the bit clock asdescribed with reference to FIG. 3. The device is further provided witha system control unit 46 for controlling the device, this system controlunit having functions corresponding to those of the system control unitof the writing device. The bit clock is coupled to the reading means 43,and is generated independently of the marks in the track. This is anadvantage when an information block subsequent to a non-written area iswritten, because the bit clock prior to the information block has thenalready been set to the correct value. In reading devices of theconventional type, the bit clock is regained from the read signal, forexample, via a PLL which then locks in on the read signal. In anembodiment of the reading device according to the invention, the clockmeans is adapted to additionally lock in on the marks. The frequency ofthe bit clock is then controlled both by the position, as in FIG. 3, andby a difference between the bit clock and the read signal of the marks.The position control has the advantage that the additional lock-in rangeof the bit clock can be limited to a great extent, because the desiredfrequency has substantially been computed and set. A correction of thephase and, if necessary, a small correction of the frequency are thenperformed by the additional lock-in on the basis of the marks. Theadditional lock-in is realized by controlling the clock means 30 onlywith the computed block and/or possibly with pulses of synchronizingelements during a jump, or when there are non-written areas. It isalternatively possible to start from synchronizing pulses derived fromthe drive means 45, such as tacho-pulses or control pulses from asynchronous motor. When the read head 52 is positioned on a written areaand marks can be read, an extra control signal is generated and appliedto the control input of the clock means. This extra control signal isgenerated, for example, by comparing the bit clock with the read signalin a phase comparator, and, based on the phase difference, the PLL isreadjusted in the clock means 30. This provides the advantage that thebit clock is substantially determined by the exactly computed valuebased on the radial position and on the track pitch information, thewinding number and the rotational speed, because the lock-in range canthereby be limited to a very large extent so that the bit clock is lesssensitive to disturbances caused by, for example, dirt on the surface ofthe record carrier.

FIG. 6 shows the calculated beat modulation. A number of numericalexamples is given, including the results of an experiment. The beatmodulation is defined as the difference between the maximum and minimumwobble beat amplitude, divided by the maximum beat amplitude. The tablebelow shows relevant numbers for existing formats and for a format(DVR-RW) according to the invention. Note that 2πt/p is smaller than 0.2in most known systems, except DVD+RW where 2πt/p=1.09, and the newproposed format of DVR-RW according to the invention (last column) where2πt/p=0.38. The range of 0.30<2πt/p<0.70 gives a significant improvementover all existing formats. CD-RW DVD+RW DVR-RW channel bit length (nm)277.7 133.3 77.0 channel bit rate (MHz) 4.3 26.2 66.0 velocity (m/s)1.20 3.49 5.08 channel bits per wobble 196 32 69 wobble length (um)54.422 4.266 5.313 wobble frequency (kHz) 22.05 817.38 956.52 trackpitch (nm) 1600 740 320 2*pi*(track pitch)/(wobble period) 0.18 1.090.38The situation described here has smallest amplitude variation of thebeat factor. This situation corresponds, however, with the highest phasevariation of the beat factor. The advantage thus depends on the relativeimportance of phase and amplitude variations.

Although the invention has been explained by embodiments using a wobblemodulation, any other suitable parameter of the track may be modulated,e.g., the track width. Also, for the record carrier, an optical disc hasbeen described, but other media, such as, a magnetic disc or tape, maybe used. It is noted that in this document, the word ‘comprising’ doesnot exclude the presence of other elements or steps than those listed,and the word ‘a’ or ‘an’ preceding an element does not exclude thepresence of a plurality of such elements, that any reference signs donot limit the scope of the claims, that the invention may be implementedby means of both hardware and software, and that several ‘means’ may berepresented by the same item of hardware. Further, the scope of theinvention is not limited to the embodiments, and the invention lies ineach and every novel feature or combination of features described above.

1. A record carrier comprising a servo track indicating an informationtrack intended for recording information blocks represented by markshaving lengths expressed in channel bits, servo track having a wobble,said wobble being a periodic variation of a physical parameter of theservo track, and the servo track constituting a concentric or spiralpattern of substantially parallel tracks at a track pitch t, wherein thetrack pitch t and a length of a wobble period p are related so that2πt/p≈n+½ where n is integer greater than zero, and wherein the lengthof the wobble period p corresponds to a whole number m times the lengthof a channel bit written on said record carrier.
 2. The record carrieras claimed in claim 1, wherein in the relation of the length of thewobble period and track pitch is 0.30<2πt/p−n<0.70.
 3. The recordcarrier as claimed in claim 1, wherein in the relation of the length ofthe wobble period and track pitch is 0.30<2πt/p<0.70.
 4. (canceled) 5.The record carrier as claimed in claim 1, wherein the number m ofchannel bits in a wobble period is
 69. 6. A recording and/or playbackdevice comprising means for writing and/or reading information blocksrepresented by marks having lengths expressed in channel bits in aninformation track on a servo track of a record carrier, said recordcarrier comprising a servo track indicating an information trackintended for recording information blocks represented by marks havinglengths expressed in channel bits, said servo track having a wobble,said wobble being a periodic variation of a physical parameter of theservo track, and the servo track constituting a concentric or spiralpattern of substantially parallel tracks at a track pitch t, wherein atrack pitch of the servo track and a length of a wobble period p arerelated so that 2πt/p≈n+½ where n is integer greater than zero, saidrecording and/or playback device comprising means for scanning the servotrack and for retrieving the record carrier information, wherein therecording and/or playback device further comprises means for generatinga wobble signal from the record carrier, and wherein the length of thewobble period p corresponds to a whole number m times the length of thechannel bit.
 7. A method of manufacturing a record carrier as claimed inclaim 1, in which the record carrier is provided with a servo trackindicating an information track intended for recording informationblocks represented by marks having lengths expressed in channel bits,said servo track being provided with a periodic variation of a physicalparameter, wherein a track pitch t of the servo track and a wobbleperiod p, p being the number of channel bits per wobble times the lengthof a channel bit, are chosen in such a way that 2πt/p≈n+½ where n isinteger greater than zero, and wherein the length of the wobble period pcorresponds to a whole number m times the length of a channel bit.